Abstract
Abstract The reported occurrence of Albian- and Cenomanian-aged braided fluvio-deltaic channels in the Orange Basin, South Africa, opens a window of exploration activities to characterize these channels as they are renowned to form some of the world’s giant oil field. In this study, a seismic acoustic impedance inversion and seismic attributes (instantaneous frequency and iso-frequency) analysis is used to investigate potential Albian and Cenomanian fluvio-deltaic channels in offshore, northern Orange Basin. Reservoirs were mapped using a well and 3D seismic volume (8-bit) after initial dip-steering coherency filtering had been performed on the seismic volume to remove incoherent noise and improve data resolution. Model-based acoustic impedance inversion was applied on the seismic volume to delineate fluvio-deltaic channels in addition to using the RMS (root mean square) amplitude attribute. Iso-frequency using the cosine correlative transform (CCT) method was equally applied to delineate these channels. Instantaneous frequency attribute was analyzed for potential hydrocarbon-charged sediments. This was achieved by utilizing thirty-three seismic traces as an input in the Hilbert transform window, after which trace envelope and instantaneous phase were transformed into instantaneous frequency. Acoustic impedance inversion results reveal the presence of two channels within the Cenomanian sequence, which shows high porosity (∼40%) along its geometry. The CCT method shows that the 8 Hz frequency window resolved the presence of a channel within the Albian sequence. A meandering channel within the Albian sequence was equally delineated by the RMS, while the application of instantaneous frequency (IF) attribute indicates the presence of hydrocarbon-charged sediments of Cenomanian age in proximity to a listric normal fault because of the attenuation of frequency observed close to the fault. This study demonstrates a case study of the application of seismic impedance inversion and seismic attributes for the delineation of potential reservoirs and hydrocarbon-charged sediments in a basin.
Highlights
Reservoir characterization studies in the Orange basin have focused more on utilizing geochemistry and petrography to appraise and predict the quality of reservoirs in the basin
The reservoirs encountered in this well are identified within the Cenomanian and the late Albian sequences, while a thinly laminated reservoir is observed just below the Albian unconformity (Figure 4)
Horizon time slice intersection on the seismic volume was performed at the top of 14AT-1 and 14JT-1 sequences in which there are occurrence of potential Albian and Cenomanian reservoirs, respectively (Figures 4 and 5)
Summary
Reservoir characterization studies in the Orange basin have focused more on utilizing geochemistry and petrography to appraise and predict the quality of reservoirs in the basin. Previous studies [1,2,3] had suggested the poor development of qualitative reservoirs due to severe precipitation of quartz and clay cements, especially in the proximal part of the Orange Basin. The aforementioned studies are more localized to specific reservoirs based on drilling campaigns and not an absolute indication of the quality of reservoirs in the basin. This underscores a need for further exploration to characterize potential reservoirs for further drilling campaign in the basin. The occurrence of fluvio-deltaic channels of Albian and Cenomanian age has been reported in the Orange Basin [5]; the delineation of these channels is critical to future exploration drive in the basin
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